For removal of odorous matter from a gaseous mixture, there have been proposed and put into practice a variety of deodorization methods, which may be broadly classified into (1) absorption methods, (2) oxidation methods, and (3) adsorption methods. However, the existing methods in these three categories are incomplete or disadvantageous from either a technical or economical viewpoint.
Odorous offensive gases are generally produced as a result of operations in industrial plants and factories in various fields and are generally in the form of a mixture gas which contains diversified matter. Most of the offensive odor-emitting matters are extremely near their threshold values at which the odors become unperceivable, and thus it appears very difficult to effect complete deodorization of a gaseous mixture which contains diversified odor-emitting matters. In this connection, according to Weber-Fechner's law, 99% removal of an offensive odor-emitting matter gives a deodorization effect as low as about 70%. Thus, the removal of an odorous matter even in the order of 99% is not considered to be practically satisfactory. It is only when a removal in the order of 99.9 - 99.99% is attained that practically feasible deodorizing effects are obtained.
It is almost impossible to carry out complete deodorization by an absorption method, in consideration of the absorption mechanism involved. With regard to the oxidation deodorization, the theoretical direct flame combustion method has a possibility of complete deodorization but it is impractical from an economical point of view. Moreover, a catalytic combustion type deodorization method has problems yet to be solved such as of that of catalyst poisoning and therefore has limitations in practical applications.
For removal of odorous matters or substances which are contained in low concentrations in a gas, an adsorption method is considered the most suitable from both theoretical and practical points of view as practically complete deodorization can be expected with a removal efficiency of 99.99% or greater. However, the adsorption deodorization method has also various problems to be solved, of which the most important one is the selection of an adsorbent which is capable of adsorbing diversified matters and which is available at a reduced production cost.
In general, matters or substances contained in gases and vapors to be adsorbed are diversified in chemical and physical properties, i.e., they may have saturated bonds, unsaturated bonds or both, may be non-polar or polar in different degrees, may have an ion bond, may be hydrophobic or hydrophilic, and may be different in particle size. Particularly in the adsorption of diversified matters or substances contained in a composite or mixture gas as in the deodorization of an exhaust gas, it is desirable to employ an adsorbent capable of simultaneously adsorbing various kinds of substances.
Industrially used solid adsorbents are broadly classified into three categories, i.e., (A) carbonaceous adsorbents, (B) silica-alumina-base adsorbents and (C) resin-base adsorbents. Representative of the carbonaceous adsorbents is active carbon and of the silica-alumina-base adsorbents is activated clay. The active carbon adsorbents are available in a variety of preparations depending upon the nature of the materials and the processes employed for the preparation thereof. The silica-alumina-base adsorbents include terra abla, activated bauxite, silica gel, alumina gel, natural and synthetic zeolite or the like. Furthermore, the resin-base adsorbent is versatile in nature depending upon the starting materials and the processes for the preparation thereof.
According to a classification by inherent selective adsorption characteristics, the carbonnaceous adsorbent belongs to a non-polar adsorbent class with slightly hydrophobic properties, and can adsorb non-polar saturated compounds almost irrespective of the kind of their functional groups, while the silica-alumina-base adsorbents belong to a hydrophilic polar adsorbent class which selectively adsorbs polar group-containing compounds and/or unsaturated organic compounds having a double or triple bond. The resin-base adsorbent class includes non-polar or highly polar adsorbents.
Among the above-mentioned adsorbents, it is only the active carbon and a certain other kind of carbonaceous adsorbent, which is imparted with a polar group by the use of granular lime, that are now used in industrial-scale gas-deodorization apparatus. The silica-alumina-base adsorbents and the resin-base adsorbent have only limited applications, due to their particular physical properties and/or high costs.
Among the non-polar adsorbents, the active carbon is considered most excellent in physical properties and lowest in cost while it is difficult to find a polar adsorbent having such excellent physical properties and low cost as compared to the non-polar active carbon.
Though synthetic zeolite or a resinous adsorbent exhibits excellent physical properties, it can not be used in industrial gas-deodorization apparatus due to its prohibitively high cost. Furthermore, application of activated bauxite, silica gel or alumina gel is limited to dehydration and removal of moisture, and terra abla is inferior in adsorptivity. In this connectin, however, activated clay which is a polar adsorbent can be industrially employed as a gas-deodorization adsorbent. Additionally, the activated clay is low in cost and, with the exception of terra abla, is available at a cost about one-third that of activated bauxite, about 1/10 that of silica gel, about 1/15 of alumina gel, about 1/15 that of activated carbon, about 1/40 that of resin-base adsorbents and about 1/100 that of synthetic zeolite.
For use on an industrial scale, the adsorbents are generally required to have, in addition to excellent adsorptivity and activity, satisfactory physical properties such as (1) particulate structure strengths, (2) abrasion resistance, (3) thermal resistance, and (4) non-disintegrability in water. However, the molded particles of activated clay usually do not meet the above-mentioned requirements (1) through (4). This is the main reason why activated clay has never been employed for adsorption in gas phase.
The fact that activated clay has not been satisfactorily used in a gas-adsorption apparatus on an industrial scale is partly attributable to noncompliance of the activated clay to the requirements or conditions imposed by existing gas adsorption apparatus.
Known gas-adsorption systems may be classified as follows:
1. A fixed-bed type system wherein gases are passed though a layer filled with a granular adsorbent. PA1 2. A moving bed type system wherein gases are brought into contact with a moving granular adsorbent. PA1 3. A fluidized bed type system wherein substances contained in a gas to be adsorbed are adsorbed while fluidizing a granular adsorbent with the gas. PA1 4. A gas flow transfer contact type system wherein a powdered adsorbent is dispersed in a gas stream and is transferred in a parallel flow with the gas stream to have odorous substances in the gas adsorbed thereinto during the transfer.
Most of the existing gas-adsorption apparatus are constructed according to any one of the above systems (1) through (3). Adsorbents to be employed in the systems (1) through (3) are invariably required to have excellent particle strength, abrasion resistance, thermal resistance and non-disintegrability in water, as described hereinbefore, due to the principles or manner of adsorption employed by these systems. However, in the gas flow transfer contact type system (4), the above-mentioned physical properties are not necessarily required since the adsorbent is used in the form of powder having a particle size of 1-150.mu. and it is possible to select an adsorbent of suitable property depending upon the kind of substances to be adsorbed.
In order to remove odorous substances successfully from a composite gas through adsorption, it has been necessary to use a number of different adsorbents which are active respectively on different kinds of substances to be adsorbed. It is, however desirable that the adsorbent have capability of adsorbing diversified odorous substances alone.
It is therefore an object of the present invention to provide an adsorbent which is particularly useful in a gas flow transfer type adsorption system for deodorization of a mixture gas.
It is another object of the present invention to provide an adsorbent which is capable of adsorbing diversified gaseous substances, i.e., hydrophobic or hydrophilic, non-polar or polar, saturated or unsaturated substances or compounds.
It is a further object of the present invention to provide a composite adsorbent which has an intensive adsorption power and a great adsorption capacity as compared with those of a unitary adsorbent.
It is a still further object of the present invention to provide a composite adsorbent which is low in cost.
It is another object of the present invention to provide a composite adsorbent which can be effectively used for deodorization of an industrial exhaust gas.